Bone loss following spinal cord injury (SCI) is an important problem in the Veteran population. Bisphosphonates are currently the frontline therapy for postmenopausal osteoporosis, but do not restore bone in patients following SCI. Thus there is a need for new therapies. Bone loss following SCI results in part from disuse, but may also result from a variety of other mechanisms including the loss of blood supply to the bone. Ding et al. have shown in a mouse model, that SCI causes a dramatic loss of bone vascular volume. We propose a preclinical rat study to determine 1) whether reduced bone blood and reduced vascular volume play a role in bone loss following SCI and 2) whether bone loss can be prevented by administration of tetramethylpyrazine (TMP) alone or in combination with passive motorized bicycle training. Our proposed study is innovative because 1) we will study bone blood supply comprehensively (i.e. we will measure both blood flow and vascular volume), 2) we will determine whether bone blood supply is compromised before SCI-induced loss of bone mineral and 3) we will test strategies to prevent both loss of bone blood supply and loss of bone mineral following SCI. Our study has 2 specific aims. Aim 1: Determine whether severe SCI causes early deficits in bone blood flow and bone vascularity and whether the time course of these changes precedes or matches that of cancellous bone loss. Hypothesis 1: SCI will cause significant loss of femoral blood flow (via in vivo microsphere administration) and femoral vascular volume (via micro CT of perfused vasculature) that will precede or accompany the reduced cancellous bone volume, reduced trabecular number, and increased osteoclast surface that we have previously reported to occur following SCI. Aim 2: Determine whether SCI-induced deficits in bone blood flow, bone vascularity, and cancellous bone volume are prevented by TMP administration or by motorized bicycle training, alone or in combination. Hypothesis 2: Administration of TMP will prevent SCI-induced changes in vasculature and cancellous bone by protecting bone blood flow. Motorized bicycle exercise will also partially protect bone as we have previously observed. We predict that the greatest protection will occur in the group receiving combined TMP and bicycle straining. Male Sprague-Dawley rats will receive a severe contusion injury vs. sham surgery. Over 4 weeks, we will assess bone blood flow in femurs by administration of microspheres to conscious rats via the femoral artery. We will also assess vascular volume in decalcified femurs of rats perfused with vascular microfil at the time of sacrifice. We will perform comprehensive analysis of cancellous bone morphology in distal femur, employing both micro CT and histomorphometry. In addition, fluorochromes will be administered to live animals to allow for histological assessment of osteoblast and osteoclast surfaces. We will administer multi- modal therapy (TMP with or without passive motorized bicycle training) to prevent bone loss following SCI. TMP is an herb-derived agent that is approved as a food additive and has been shown to protect bone following glucocorticoid administration. Our preliminary data shows that passive bicycle training partially protects bone following SCI.